The three-subunit cytochromebc 1 complex ofParacoccus denitrificans. Its physiological function,structure, and mechanism of electron transfer and energy transduction

The cytochromebc₁ complex purified fromP. denitrificans has the same electron-transfer and energy-transducing activities, is sensitive to the same electron-transfer inhibitors, and contains cytochromesb, c₁, iron-sulfur protein, and thermodynamically stable ubisemiquinone identical to the counterpart complexes from mitochondria. However, the bacterialbc₁ complex consists of only three proteins, the obligate electron-transfer proteins, while the mitochondrial complexes contain six or more supernumerary poly-peptides, which have no obvious electron-transfer function. TheP. denitrificans complex is a paradigm for thebc₁ complexes of all gram-negative bacteria. In addition, because of its simple polypeptide composition and apparently minimal damage during isolation, theP. denitrificans bc₁ complex is an ideal system in which to study structure-function relationships requisite to energy transduction linked to electron transfer.